A population that belongs to the same species, but differs from other populations in terms of a certain trait as a result of having a different genetic composition, is referred to as a cultivar. In other words, even within the same species of plant, cultivation difficulty, resistance to damage caused by diseases and insects, yield, quality and the like differ according to the particular cultivar. Consequently, in agricultural products and particularly in major crops such as rice, barley or wheat, cultivar improvement has been carried out extensively in order to obtain better cultivars, and in recent years, cultivar improvement has been aggressively implemented by not only nursery companies and other private firms, but also by government agencies at the national and prefectural levels.
Accompanying recent progress made in fields such as nucleic acid analysis technology, the genes of various plants such as thale cress, rice and wheat have been analyzed, and the resulting genetic information has been disclosed. Cultivar improvement consisting of introducing a gene from an introduced species using genetic recombination methods is being carried out extensively by using this disclosed genetic information. For example, Hd1 gene, which encodes a plant-derived protein that has the function of increasing plant photosensitivity, and a method for producing a transgenic plant into which Hd1 gene has been introduced, have been disclosed (see, for example, Patent Document 1). However, although cultivar improvement by genetic recombination has the advantage of being able to introduce a trait possessed by a distantly related species for which crossbreeding is normally not possible, there is the problem of not always being able to adequately verify the safety thereof.
Consequently, new cultivars are being extensively produced by non-genetic recombination methods in the case of edible plants including rice. For example, Patent Document 2 discloses a method for producing a new cultivar having a target trait, without altering preferable traits possessed by the original cultivar, by controlling a substitution region using a chromosome fragment derived from an introduced cultivar in the case of substituting with an exogenous useful chromosome fragment by a non-genetic recombination method. The same Patent Document 2 also describes a new rice cultivar in the form of Koshihikari eichi no. 3 in which only a region containing Habataki Hd1 gene is introduced into Koshihikari by this method for producing a new cultivar.
In rice in particular, Koshihikari is desired that is able to be cultivated over a wider range of regions. Koshihikari has flavor that is superior to that of other cultivars and is preferred by consumers. Consequently, rice farmers cultivate Koshihikari preferentially even in regions that cannot always be said to be suitable for cultivation of Koshihikari. However, in the case of cultivating Koshihikari in southern regions, a satisfactory yield cannot be expected due to the heading time being excessively early. Moreover, flavor ends up decreasing due to persistent high temperatures during the heading time. On the other hand, in the case of cultivating Koshihikari in northern regions, it ends up maturing late, and even if heads have appeared, for example, their ripening is poor due to low temperatures, thereby preventing rice from being harvested.